(1) Field of the Invention
The present invention relates to an image scanning technology for recording or reading an image on a surface by scanning the surface with a light beam by use of a deflecting device such as a polygon mirror and a scanning lens, especially to an image scanning method and apparatus for compensating for a pyramidal error of the polygon mirror by fine-adjusting an angle of the light beam incident on the polygon mirror.
(2) Description of the Prior Art
In a flatbed scanning prepressing scanner, a light beam for image reading or recording is required to be deflected along a main scanning direction thereof periodically. Generally used as a deflecting device is a polygon mirror.
A locus of the light beam deflected by the polygon mirror on a plane (namely, a line made of dots formed on the plane in the main scanning direction; will be referred to as a trace line) is not necessarily a completely straight line, but tends to be deviated in a sub scanning direction or curved due to a pyramidal error of the polygon mirror.
The above phenomena, especially if they are excessive, deteriorate image quality or reading precision. In order to prevent such malfunction, the pyramidal error is compensated for.
Due to the problems inherent in manufacturing a polygon mirror, it is not avoidable that the parallelism of each mirror surface of the polygon mirror is nonuniform with respect to a rotating axis of the polygon mirror. The pyramidal error caused by the non-uniformity is, for example, .+-.10".
Japanese Patent Publication Kokai No. 58-100117 has disclosed an image scanning apparatus for compensating for the pyramidal error. A laser beam is incident on a polygon mirror from a main scanning plane. (The main scanning plane is a deflecting plane perpendicular to a rotating axis of the polygon mirror and includes an optical axis of an f.multidot..theta. lens for focusing the light beam reflected on each mirror surface of the polygon mirror and leading the focused light to a photoconductive body.) The angle of incident of the laser beam is fine-adjusted by an AOD (acoustic optical deflector) or the like in accordance with a pyramidal error of each mirror surface and a rotating angle of the rotating axis. More practically, the laser beam from a main scanning plane is deflected by an angle corresponding to a pyramidal error of the polygon mirror in such the direction that counterattacks the deviation or the curve of the trace line. As a result, the light beam reflected on the polygon mirror is surely reflected on the main scanning plane, whereby the trace line is prevented from being deviated or curved.
In the above apparatus, when the polygon mirror having a pyramidal error is rotated, the angle made of the incident light beam and the polygon mirror is constantly changed due to the scanning characteristic of the f.multidot..theta. lens employed as the scanning lens. The trace line is asymmetrically curved as shown with a dashed line in FIG. 1 with respect to the ideal sub scanning line indicated by the 4 axis. This means a deviation amount of the trace line with respect to the ideal sub scanning line is different depending on at which position of the mirror surface the light beam is reflected.
Such a problem is solved by detecting the rotating angle of the rotating axis of the polygon mirror as well as the pyramidal error, and fine-adjusting the angle of incidence of the laser beam continuously for each mirror surface in accordance with the rotating angle and the pyramidal error. Since this requires quite fast and precise control, the control system gets complicated, huge and expensive.
Japanese Patent Publication Kokai No. 60-186821 has also disclosed an image scanning apparatus for compensating for the pyramidal error. A laser beam is incident on a polygon mirror from right below of an optical axis of an f.multidot..theta. lens which is provided outside a main scanning plane and on an outgoing side of the polygon mirror. The angle of incidence of the laser beam is fine-adjusted by an AOD or the like in accordance with the pyramidal error of each mirror surface as in the conventional apparatus mentioned above. In this way, the trace line is prevented from being deviated or curved.
The above construction does restrict the deviation or the curve of the trace line. However, the incident laser beam has an angle with respect to the optical axis of the f.multidot..theta. lens since the main scanning plane is off the optical axis. Therefore, the trace line runs farther and farther from the ideal sub scanning line to be a linebow as shown with a solid line in FIG. 1. Such a phenomenon deteriorates image quality and reading precision. The apparatus disclosed in this publication is equipped with an optical compensating device such as a concaved mirror. If the angle made of the incident light beam and the optical axis of the f.multidot..theta. lens is large (for example, 5.degree. to 15.degree.), the curve of the trace line is too large for practical use. Such a large curve is only compensated by a concaved mirror having a width at least equal to the scanning width of the laser beam, which makes the optical system complicated and huge and expensive.